LIGNIFICATION 283 



cells with open communication between them, owing to 

 the cross walls having been dissolved wholly or partially 

 by the protoplasm before the cells were fully developed. 

 A vessel is thus comparable with a sieve tube in that 

 both are conducting structures . formed of a chain of 

 cells in continuity with one another. Vessels may be 

 pitted, scalariform or reticulate, and have exactly the 

 same function as chains of tracheids. They are, however, 

 more efficient than chains of tracheids because, owing 

 to the central perforation or complete disappearance 

 of the cross walls, they offer less resistance to the 

 passage of water. The wider water-conducting elements 

 are always vessels. The widest tracheids are not more 

 that 100 jLi in diameter, usually much less, while vessels 

 commonly reach a width of 300 /x, and some measure 

 as much as 700 n across. 



Alteration of the Substance of the Cell Wall. — The 

 walls of the living cells of the plant body commonly 

 remain cellulose, but in many cell walls important 

 changes affecting the functions of the cell takes place 

 in the wall, usually while the cell is still alive, but 

 sometimes after death. 



One of the most important modifications is lignification, 

 Lignification occurs in its extreme form in the walls of 

 the tracheids, vessels, wood fibres, and often in the paren- 

 chyma of the wood. It also often affects other fibres to 

 a greater or lesser degree. It depends on the deposition 

 in the wall of Ugnocellulose, consisting of cellulose and 

 two other constituents (an aromatic substance and a 

 pentosan) often classed together as lignin. This gives 

 characteristic colour reactions with various chemical 

 reagents, such as phloroglucin and hydrochloric acid 

 (magenta red), pyrogallol and hydrochloric acid (blue- 

 green), aniline chloride or aniline sulphate (golden 



